I like this group for its use to promote discussion and as a provocation at the beginning of the unit. Some obviously are not school appropriate images so use good judgement.
https://www.facebook.com/HistoryInPictures/I participate each year in the Global Read Aloud and love the connections it makes for both the students and the teachers.
https://www.facebook.com/GlobalReadAloud/Teach Thought is always a good reference for information and resources.
https://www.facebook.com/teachthought/For those using Note and Notice by Kylene Beers you will love this page for access to resources and other help from fellow teachers using these strategies.
https://www.facebook.com/search/top/?q=notice%20and%20note